(1) Field of the Invention
The present invention relates to a method of determining the static force developed by a servo-control.
(2) Description of Related Art
Conventionally, an aircraft has controllable members that can be operated by a pilot, such as the blades of a lift rotor in a rotorcraft of the helicopter type, or indeed the control surfaces of an airplane, for example.
By using flight controls, the pilot thus operates the controllable members of the aircraft. Nevertheless, the forces that need to be exerted in order to move these controllable members are sometimes very large.
Consequently, the linkage connecting a flight control to a controllable member is often provided with a hydraulic system that includes a powered servo-control so as to enable the pilot to pilot the aircraft accurately and without difficulty.
More particularly, a helicopter is provided with a main rotor providing it with lift and propulsion. In order to control the flight of the helicopter, a pilot modifies the pitch of the blades of the main rotor, i.e. the angles of incident of the blades relative to the incident air flow.
As a result, the rotorcraft has a swashplate assembly provided with a non-rotary bottom swashplate and with a rotary top swashplate, with said assembly sometimes being referred to more simply as the “swashplate”. The non-rotary bottom swashplate is connected to the pilot's flight controls, generally by three distinct lines, while the rotary top swashplate is connected to each of the blades by a respective pitch-control rod. The swashplate thus slides along the mast of the main rotor in order to control the general pitch of the blades of the main rotor, while also being capable of oscillating in all directions around a ball joint in order to control the cyclic pitch of the blades.
The oscillations and the vertical movement of the swashplate, under the control of the pilot, are thus at the origin of variations in the pitch of the blades that enables the pilot to control the helicopter.
Conventionally, the pilot controls the swashplate via mechanical controls that are connected thereto by connecting rods. Nevertheless, the forces the pilot needs to exert in order to move the swashplate are very large, in particular when the rotorcraft is a heavy rotorcraft.
Consequently, a hydraulically-powered servo-control is arranged between an upstream portion and a downstream portion of each control linkage. The pilot then acts on the servo-controls without exerting large amounts of force via the upstream portion, and the servo-controls then transcribe the orders from the pilot and act on the downstream portion of the linkage.
Similarly, a helicopter is provided with a tail rotor and the pitch of its blades can be modified by means of a servo-control.
Naturally, the same applies of the ailerons or the flaps of airplanes, which may be themselves operated by servo-controls.
It should be observed that certain modern aircraft have electric flight controls that replace mechanical connections connecting the flight controls to the servo-controls.
In conventional manner, servo-controls include an actuator having at least one outer member of cylindrical shape in which a slidable element moves in translation, said slidable element having a power rod carrying a control piston. The control piston defines a retraction chamber and an extension chamber inside the cylinder constituted by the outer member.
Furthermore, the servo-control includes a hydraulic distributor control valve that feeds fluid to the retraction chamber or to the extension chamber depending on the order it has received. The movement of the control piston of the slidable element relative to the outer cylinder is then controlled by the hydraulic distributor control valve, which is actuated by the flight controls of the helicopter pilot, acting via the upstream portion of a linkage. Depending on the orders given, the hydraulic distributor control valve feeds hydraulic fluid to the retraction chamber or to the extension chamber in order to request retraction or extension of the servo-control.
It will be understood that the term “retraction chamber” is used below to mean a chamber that causes the servo-control to retract when said chamber is filled with a fluid. Conversely, the term “extension chamber” is used to designate a chamber that causes the servo-control to extend when said chamber is filled with a fluid.
The servo-control may also include slaving means, optionally incorporated in the hydraulic distributor control valve.
Two servo-control configurations then coexist.
In a first configuration, the power rod is fastened to a stationary point of the aircraft, e.g. forming part of a main gearbox, with the cylinder moving as a function of the orders received and being connected to the downstream portion of the linkage. The person skilled in the art refers to that kind of servo-control as a “moving cylinder servo-control”.
In contrast, in a second configuration, the cylinder is fastened to a stationary point of the aircraft, and it is the power rod that moves as a function of the orders received, and that is connected to the downstream portion of the linkage. The person skilled in the art then refers to that type of servo-control as a “stationary cylinder servo-control”.
In addition, whatever the embodiment, the person skilled in the art is aware of so-called “single” cylinder servo-controls and so-called “dual” cylinder servo-controls.
A single cylinder servo-control then has an actuator provided with a cylinder defining a single inside space defining a retraction chamber and an extension chamber, which chambers are separated by a control piston. The retraction chamber and the extension chamber are then fed by a hydraulic distributor control valve that has a single hydraulic unit. The servo-control performs its function perfectly well. Nevertheless, for safety reasons and, above a certain level of force that needs to be developed, the person skilled in the art tends to make use of a servo-control having at least two cylinders.
A dual cylinder servo-control then has an actuator provided with a lower cylinder and an upper cylinder that are assembled together in tandem or in parallel.
For example, a tandem dual cylinder servo-control comprises a sliding element having a power rod carrying two pistons, each piston defining a retraction chamber and an extension chamber in a respective one of the cylinders.
Furthermore, two hydraulic units of the hydraulic distributor control valve that are actuated by a common control lever connected to the pilot's controls serve to feed the retraction and extension chambers of the lower and upper cylinders, respectively.
There also exist cylinders that have three or even more cylinders.
During a flight at high speed, extreme maneuvers of the aircraft may give rise to high levels of mechanical stress in the load-bearing structure of the aircraft. Beyond given load factors, there is a risk of damaging the structure.
In order to warn the pilot that the aircraft has reached a maneuvering limit, it is possible to provide a device for detecting a limit static force on a servo-control. When the static force exerted on the servo-control reaches a limit threshold, i.e. a traction static force or a compression static force, then the limit static force detection device triggers a warning to inform the pilot.
Conventionally, the limit static force detection device comprises a detector element having a rod fitted with a detection piston that slides in a detection space, the detection space comprising two detection chambers that are separated by the detection piston and that are independent of the retraction and extension chambers of the outer cylinder. The first detection chamber is fed with fluid by the hydraulic circuit of the aircraft, while the second detection chamber opens out to the outside of the servo-control.
In addition, the rod of the detection element projects from the cylinder of the servo-control so as to be connected to the downstream portion of the linkage, for example. This projecting portion of the detection element also includes a lever suitable for co-operating with a pushbutton switch.
Below the limit threshold, the pressure that exists in the detection chamber holds the detection piston in a high abutment position so as to keep its lever away from the switch. In contrast, when the threshold is reached, the pressure that exists in the detection control can no longer hold the detection piston in the high abutment position. The detection piston then reaches a low abutment position with the lever then actuating the switch.
In order to avoid fluid passing from the first detection chamber to outside the servo-control, the detection piston includes a sealing ring or gasket. Since the gasket is stressed dynamically, it is possible that leaks to the outside of the servo-control will appear and will require maintenance action.
Furthermore, the limit static force detection device is subjected to the forces to which the servo-control is subjected by being connected to the control linkage. Under such circumstances, it is dimensioned so as to be capable of withstanding said forces. This results in non-negligible costs and weight.
Finally, the sliding of the detection piston gives rise, in reality, to slack in the control linkage in the event of a drop of pressure in the hydraulic circuit feeding fluid to the limit static force detection device.
A servo-control is also known that has at least one limit force detection device. That device comprises a casing secured to the cylinder of the servo-control, the casing defining a detection space. Furthermore, a movable member subdivides this detection space into a first detection chamber opening out to an inside space of said cylinder, and a second detection chamber. Finally, the device possesses means for detecting the position of the movable member in the detection space.
The state of the art also includes Document WO 2008/095525, Document US 2004/0128868, and Document US 2010/0294125.